Cryovolcanoes on Ceres

It’s been a little while since the Dawn probe imaged those mysterious ‘lights’ in the craters of the dwarf planet Ceres (1). On first impression, these seem to be impact marks where brighter materials lying below the surface were exposed following meteoritic bombardment. But they are uncommonly bright for an asteroid, so speculation about the nature of the materials involved has been rife in the planetary science community, and what it could mean for how the dwarf planet formed in the first place (2). The bright spots, now widely thought to be salt deposits, have recently even been given names:

“The two most famous bright spots on Ceres have been given names. These once-mysterious spots are now thought by most scientists to be salt deposits. They’re now called Cerealia Facula (for the brighter of the two spots) and Vinalia Faculae (for the cluster of less reflective spots to the east). Both names are related to ancient Roman festivals.” (3)

But that’s not the only mystery on Ceres. There is a growing consensus that there may be geophysical processes going on that are relatively recent (at least in terms of geological time periods):

“Dawn’s cameras revealed that the floors of several craters on Ceres are coated with flowing material akin to lava on Earth. But the material looks less viscous than similar formations elsewhere in the solar system, [Ralf] Jaumann [of the German Aerospace Center] says, and it is tinged slightly blue – the same color as a mountain on Ceres thought to have formed by eruptions of ice mixed with mud. Jaumann and his colleagues theorize that the flowing material is a mixture of ice, mud and salts that erupted out of weak points in Ceres’ surface when large space rocks or other objects bombarded the dwarf planet. He and his team will publish their results in an upcoming issue of Geophysical Research Letters.

“Dawn also found Ceres is loaded with “vast deposits of ice” in its upper layer of soil, says Thomas Prettyman of the Planetary Science Institute. In Ceres’ polar regions, the ice is right up at the surface, Prettyman and his colleagues report in this week’s Science. The ice levels make clear that Ceres must have had liquid water at some point, Prettyman says. The new findings of ice on Ceres are “reliable,” says Jennifer Scully of the Jet Propulsion Laboratory at the California Institute of Technology, who was not involved with the research. And the flows on Ceres are young in geological terms, she says, showing Ceres is not just a static world but a place of change and movement.” (4)

New findings, presented at the American Geophysical Union meeting in San Francisco this month, show that ice can happily exist in permanently shadowed craters on Ceres and is widespread below the surface (5). in fact, Ceres is something of an ‘ice palace’. This has raised the question of whether Ceres might have an abundance of active ‘cryovolcanoes’ lurking in the darkness of some of its craters, despite the extremely cold temperatures of its general environment (4). Furthermore, there appears to be spectrum of colour for the Cerean ices, ranging from a bluish tinge for the recently exposed/erupted sub-surface ices to a more reddish tinge for the older crater ices (6). It’s not known why this colour differential is evident, but it seems to imply some kind of process at work on the surface of Ceres. Cryovolcanic activity is the mostly likely theory to explain this effect.

The counter-argument that this theory has encountered in the past is that there needs to be an internal source of heat and pressure to drive these volcanic processes, which on the face of it seems unlikely on an isolated planet of the size of Ceres. Water vapour was seen emerging from Ceres back in 2014, and it was thought that this effect might be due either to (a) cryovolcanoes, or to (b) the sublimation of ice to water vapour (in a similar fashion to that of comets) (7). Both possibilities have their merits and problems.

It’s looking increasingly like Ceres may indeed be more geophysically active than previously thought possible for an isolated dwarf planet located in the asteroid belt. Back in September, news emerged of a 13,000′ high ice mountain on the surface of Ceres, named Ahuna Mons, which is thought to be an ice volcano:

“Giant mountain Ahuna is a volcanic dome built from repeated eruptions of freezing salty water.” (8)

It seems as though Ceres has an abundance of water ice (9), much of it lying close to its surface, and some of it wrapped up in this huge natural structure. One of the implications is that in the past liquid water once flowed over the Cerean surface. This seems eerily reminiscent of similar arguments about Mars, whose own atmosphere is too weak to sustain liquid water on the planet’s surface. Yet, evidence suggests that water has indeed flowed across the red planet’s surface. So, might Mars also have cryovolcanoes, covered in a heavy dusting of regolith? Might similar processes on Mars send plumes of water vapour into its atmosphere, thus falling back as freezing rain, becoming flash-flood channels upon the surface? This may well explain what’s seen on Mars.

Looking at the bigger picture, the solar system is providing us with evidence of more active geophysical processes than previously thought possible. Below the seemingly dull surfaces of various worlds, there is quite a lot going on. This flies in the face of theory, which previously considered these worlds to be essentially dead lumps of rock. Migration of planets seems a key factor, helping to explain why planets located within the snowline have so much water ice buried beneath the surface (which should have been driven off early in the lifetime of the solar system). This opens up the possibility that the Earth, too, may have migrated from a different location further away from the Sun. Remember – the Earth also has a rather extensive water resource given its current relative proximity to the Sun. Water is also evident on the Moon. Now available abundantly on Ceres. Probably, then, under the surface of most asteroids?

Isn’t it ironic that comets aren’t ‘dirty snowballs’ as had been predicted for decades, but rocky bodies with internal volatiles? At the same time, asteroids may not be dead rocks after all, but complex, water ice-infused structures which include the remarkable ‘ice palace’ Ceres. The entire fabric of the solar system is testing the old and tired theory of steady-state ennui. Radical hypotheses are required to explain this about-turn.